The present invention relates to a spread spectrum communication system and an overload control method for a case wherein the interference amount of a reverse link signal in communication between a mobile station and a base station in a cell (communication from the mobile station to the base station) is equal to or larger than an allowable value in the spread spectrum communication system.
In a spread spectrum communication system, a plurality of communications can be performed by using the same frequency band. This is because each communication is spread-modulated with codes having orthogonality on the transmission side, and each communication can be specified by spread-demodulation (despreading) with the same codes on the reception side.
This orthogonality is made imperfect by propagation delay differences due to geographical and weather conditions and the like and time deviations due to multipath in the propagation path between the mobile station and the base station, and multipath associated with irrelevant codes, i.e., irrelevant communication, and multipath associated with relevant codes, i.e., relevant communication, have correlation components in some case. These correlation components become interference components in the relevant communication, resulting in a deterioration in communication quality. Since interference components are generated by such a factor, interference components increase as the number of communications increases.
In a radio communication system, a predetermined signal/noise ratio is generally required to ensure high communication quality. In a spread spectrum communication system, since each communication is performed by using the same frequency band, a predetermined signal/(noise+interference) ratio is required to ensure high communication quality. This ratio is generally expressed as a signal/interference ratio=Eb/IO ratio (Eb: desired reception wave power, and IO: interference wave power). In addition, the Eb/IO required to ensure predetermined communication quality is expressed as a specified Eb/IO ratio.
In general, communication quality is determined on the basis of a frame error rate (FER). The Eb/IO ratio required to obtain a given FER changes owing to the influences of fading frequencies and Eb/IO ratio measurement accuracy, and hence needs to be always corrected. For this reason, a base station changes the Eb/IO ratio specified to obtain the FER required to guarantee communication quality in accordance with the measured FER of a received radio wave such that the FER required to guarantee the communication quality remains constant. This function is called outer loop control.
In general, as the number of communications increases, since the interference wave (IO) increases, the Eb/IO ratio decreases. In this case, a base station increases the transmission output of each mobile station to increase the signal level (Eb) so as to ensure the specified Eb/IO ratio in each communication.
In a spread spectrum communication system, however, since excessive signal energy causes interference in irrelevant and relevant communications, the Eb/IO ratios in the respective communications in the base station must be made almost equal. The base station therefore controls the transmission power of each mobile station to make the Eb/IO ratios in the respective communications almost equal. This control is called high-speed closed loop control because the control operation is performed at high speed by feedback control between a mobile station and a base station to absorb level variations such as fading.
An Eb/IO ratio as a reference in high-speed closed loop control will be referred to as a reference Eb/IO ratio. In general, this reference Eb/IO is set to be equal to the specified Eb/IO. That is, the Eb/IO ratio specified to obtain an FER that guarantees communication quality and obtained by outer loop control is set to the reference Eb/IO ratio in high-speed closed loop control, and each mobile station is instructed to increase/decrease the transmission power such that the Eb/IO ratio obtained by reception power from each mobile station is set to this reference Eb/IO ratio in the base station.
A combination of this outer loop control and high-speed closed loop control will be referred to as transmission power control.
As shown in FIG. 10A, however, when the communication amount becomes a predetermined amount or more, even if the signal level (Eb) is increased, the interference amount increases accordingly. As a result, as shown in FIG. 10B, the specified Eb/IO cannot be obtained. In this case, if the communication amount further increases, all communications are disabled. This state is a state in which the interference amount is equal to or larger than the allowable value.
Another problem is that the base station continuously increases the transmission output of each mobile station to obtain the specified Eb/IO ratio in each communication, and unnecessarily large interference is caused in neighboring base stations as well. As a result, the communication capacities of the neighboring base stations decrease. In the worst case, communication failures occur in the neighboring base stations. Furthermore, a chain reaction occurs in such a manner that communication quality deteriorates or communication failures occur in a wide range.
In a conventional technique (IS-95 system), that the reverse link interference amount between communications in a cell is equal to or larger than an allowable value is detected in accordance with a decrease in FER of reverse link radio frames owing to a failure to obtain the specified Eb/IO ratio. On the basis of this detection result, origination/termination control is performed for the base station in which the reverse link interference amount is equal to or larger than the allowable value and mobile stations belonging to the neighboring stations, thus reducing a deterioration in communication quality.
To increase the FER measurement accuracy, however, many samples are required, and hence it takes much time to detect such a deterioration. For this reason, the transmission power of each mobile station belonging to the base station in which the reverse link interference amount between communications in the cell is equal to or larger than the allowable value is further increased until a deterioration is detected. Meanwhile, excess interference occurs in the neighboring base stations.
In the above conventional spread spectrum communication system, it takes much time to detect that the interference amount between communications in a cell is equal to or larger than the allowable value, because this detection is based on detection of a decrease in FER.
It is an object of the present invention to provide a spread spectrum communication system which can detect in a short time that the reverse link interference amount between communications in a cell is equal to or larger than an allowable value.
In order to achieve the above object, according to the present invention, there is provided an overload control method for a spread spectrum communication system, comprising the steps of, when the number of mobile stations in the process of communication with a base station in a self-cell is not less than a predetermined number, measuring a ratio of increase instructions to reverse link transmission power control bits contained in radio frames transmitted to the mobile stations from the base station and indicating increases/decreases in transmission power of the mobile stations, when the measured ratio of increase instructions is not less than a predetermined ratio, determining that a reverse link interference amount between the base station and the mobile stations is not less than an allowable value, and when it is determined that the reverse link interference amount between the communications is not less than the allowable value, reducing the reverse link interference amount between the communications by performing at least one of transmission output restriction and origination/termination restriction.